Flexible riser system

ABSTRACT

Riser tower system connecting a fixed subsea installation to a floating surface unit, comprising at least one flexible pipe arranged in a first catenary and extending between the surface installation and a submerged buoy; at least one riser arranged in a second catenary between the buoy and the seabed, the buoy being anchored to the seabed by a tether device comprising at least two taut tethers, at least two moorings in a catenary and on which there are provided a return device which exerts on the buoy a return force that depends on the lateral movement of the buoy. It has oil industry applications for offshore operation.

The present invention relates to a riser tower system intended to connect a fixed subsea installation such as a well head or a manifold to a floating surface unit such as a platform or a ship of the FPSO (Floating Platform Storage and Offloading) type.

Offshore operation of an oil field is increasingly complicated as the water depth increases, which depth these days may be as much as several thousands of metres. The transfer of product from the fixed installation, situated on the seabed and usually consisting of a well head, to the floating surface installation or unit, poses a certain number of difficulties. The most commonly used transfer systems are those known as riser tower systems which comprise pipes through which various products to be transported between the seabed and the surface flow, these products being, for example, oil, gases, water, etc. Other pipes may also be used, particularly fluid-injection, charging or electrical and/or hydraulic control lines.

In drilling for oil, particularly when the deposits are very deep down, the areas of turbulence which lie between 50 and 300 m below the surface of the water may have effects not only on the surface unit or installation which may move as a result of the swell and of other phenomena such as pitching, rolling, etc., but may also have an influence on the riser tower system which experiences forces due to the waves, to the wind and to marine currents.

As a result, riser tower systems are designed to withstand these stresses the magnitudes of which may vary.

Various types of riser tower system have been proposed, these are described for example in U.S. Pat. NoS. 3,111,692, 3,677,302, 4,031,919, 4,188,156, 4,182,584, 4,388,022, 4,400,109 and 4,423,984.

The main disadvantages with these systems lie in the fact that it is necessary to use buoys of great buoyancy of at least 2000 tonnes when this buoyancy is distributed over the entirety of the riser, the elements of the buoy having to withstand significant pressures. Another disadvantage is that these systems are manufactured on land and have then to be brought out to and installed on the site, all of these operations being tricky and expensive. In addition, it is very difficult to anchor the ends of the rigid sections to the seabed without using divers or very sophisticated equipment such as ROVs (Remote Operated Vehicles), and this introduces not insignificant costs into the placement and monitoring while the field is being exploited.

U.S. Pat. No. 5,639,187 describes a system which combines rigid pipes and flexible pipes to establish the fluid communication between the fixed subsea installation and the surface unit, the system comprising a submerged buoy which is anchored to the seabed by means of four taut tethers, each of the tethers being attached to the ends of the buoy and to a corner of a kind of rectangle formed on the seabed, so as to minimize the rotation of the buoy that could be brought about by horizontal forces and by the weight of the pipes running between the buoy and the seabed. In fact, the system is of the tension leg type, these having to withstand vertical loadings of at least 1500 tonnes and even more in order, especially during installation, to react the buoyancy of the buoy which has to be greater than the weight of the pipes, of which there are a great many in such an application. The submerged buoy has also to have a reserve of buoyancy so as to give the entire system the stiffness it needs to limit its lateral movements. What happens is that the lateral movements of the buoy are undesirable because they bend the rigid pipes to bend radii such that these pipes may experience plastic deformation and thus cause the beginning of crushing by ovalization at the sag bend. The sag bend lies over the touchdown point which is where the pipes touchdown onto the seabed.

The purpose of the present invention is to propose at least a riser tower system which is better suited to the movements of the surface unit, simple to produce and to install and less expensive than the systems of the prior art, for comparable operating conditions.

One subject of the present invention is a riser tower system intended to connect a fixed subsea installation to a surface unit, of the type comprising at least one flexible pipe arranged in a catenary and extending between the surface installation and a submerged buoy, at least one riser arranged in a catenary between the said buoy and the seabed, the said buoy being anchored to the seabed by a tether device comprising at least two taut tethers, and which is characterized in that it comprises at least two moorings in a catenary and on which there are provided return means which exert on the said buoy a return force that depends on the lateral movement of the said buoy.

One advantage of the present invention lies in the fact that, when the submerged buoy moves laterally, it is automatically returned to its initial position or position of equilibrium by the return means the force of which is variable, that is to say that they develop a return force which is dependent upon the amplitude of the lateral movement of the buoy.

According to another feature of the invention, the return means consist of a ballast weight which is distributed on each side of the touchdown point where the mooring touches down on the seabed. The weighted part of the mooring, situated above the touchdown point, in the direction of the buoy, mainly constitutes the return force. Such a structure is simple to produce because the ballast weight can be of any kind, such as chains, balls, weights or alternatively clump-weight. In addition, it is easy to determine the “return” length of the mooring, that is to say the length of the weighted part, in advance. Finally, it is possible to weight down a longer length than necessary of the portion of the mooring lying along the seabed if it is desirable not to anchor the end of the mooring. In such a case, care should be taken that the mooring does not move excessively and does not become entangled in the buoy's taut tethers.

According to another feature, each mooring is connected to the buoy by a flange or mooring bridle arranged under the buoy and intended to prevent or at the very least limit the rotation of the buoy.

Other advantages and features will become apparent from reading the description of one embodiment of the present invention and from the appended drawings in which:

FIG. 1 is a schematic view in elevation of the riser tower system according to one embodiment of the invention.

FIG. 2 is a schematic view in elevation of the riser tower system for several lateral positions of the buoy.

The riser system 1 depicted schematically in FIG. 1 is intended to connect a fixed subsea installation 2, consisting for example of a well head, a manifold or some other collector and delivering a product from an oil deposit or the like, to a floating surface unit or installation 3 such as a platform or an FPSO, the distance separating the surface 3 and subsea 2 installations being possibly as much as several thousands of metres. At a certain distance from the surface 4 of the water, and generally beyond the area of turbulence of the stretch of water concerned, is submerged a buoy 5 which is anchored to the seabed 6 by two tethers 7, 8 which are stretched between the buoy 5 and a deadweight 9 or other anchoring means (suction pile).

One or several flexible pipes 10 running in a catenary between the surface unit 3 and the buoy 5 are connected to one or several risers 11 extending in a catenary between the buoy 5 and the fixed subsea installation 2 so that fluid communication is established between the said installations 2 and 3. The risers running in a catenary from the buoy on the seabed may be of any type such as rigid pipes commonly known as SCRs (Steel Catenary Risers), single-walled or pipe in pipe and even flexible pipes or hybrid pipes having at least one flexible part and one rigid part.

The riser tower system comprises at least two moorings 18, 19 arranged in a catenary between the buoy 5 and the seabed 6. Each mooring 18, 19 comprises, in the lower portion 14, a part 13 which is weighted by a ballast weight 12. This ballast weight 12 constitutes return means for the buoy, the return force being dependent chiefly upon the lateral movement that the said buoy 5 might have and which may be caused by a strong swell, marine currents and more generally by movements of the surface unit 3. The ballast weights 12 are distributed on each side of the touchdown point 15 which is the region or point where the mooring touches down on the seabed 6. The ballast weights 12 may consist of weights, balls, chains or clump-weight. The ballast weights are distributed on each side of the touchdown point when the buoy is in its position of equilibrium (central position A in FIG. 2).

In the position A which corresponds to that of FIG. 1, the tethers 7, 8 are roughly vertical and the weighted parts 13 of the moorings 18, 19 rest for the most part on the seabed 6. In the position C, the weighted parts 13 rest more on the seabed whereas in the position B, the weighted parts 13 are lifted and thus develop a return force which has a tendency to return the system towards the position A, the return force varying according to the weighted length raised off the seabed by the movement of the corresponding mooring, which movement is brought about by the lateral movement of the buoy 5 (FIG. 2).

By way of example, the ballast weight on each mooring 18, 19 consists of 4-inch (≅10 cm) chains spread over a 100 m, in the knowledge that when the buoy 5 is at the mid-point (vertical), about one third of the ballast weight is raised off the bed and produces a tension of about 50 tonnes in each mooring 18, 19. Obviously, these indications are given solely by way of example, the choice and arrangement of the elements of which the ballast weight is made being dependent on the particular case envisaged. However, it is possible to suggest that the weight per unit length used to weight the catenary mooring 18, 19 is dependent in particular on the distance between the buoy 5 and the seabed 6.

Each taut tether 7, 8 is attached to a comer 16 of a flange or mooring bridle 17 fixed to the buoy at one end thereof, and which is also intended to prevent or greatly limit the rotation of the buoy. The points of attachment of the tethers 7, 8 to the flanges 17 are preferably roughly in the midplane passing through the longitudinal axis 18 of the buoy 5. In FIG. 1, the midplane comprising the tethers 7, 8 is embodied in dotted line X-X.

Each mooring 18, 19 is connected to an end 20 of the buoy 5 which is the opposite end to the other lateral end 21 to which the riser or risers is or are connected. It may also be connected to the top 16 of the flange 17 so that the points of attachment of the moorings 18, 19 lie more or less in the midplane in which the points of attachment of the moorings 7, 8 to the buoy 5 are situated.

In a preferred embodiment of the invention, the buoy 5 has variable buoyancy and comprises several parts, for example three parts 22 to 24 each consisting of a hollow cylinder. Such a structure of the buoy 5 makes it possible to avoid having to develop very high forces at the buoy 5, which forces are dependent in particular on the number and weight of risers to be provided between the seabed 6 and the said buoy 5. Indeed, by virtue of this compartmentalization of the buoy 5, each cylinder 22 to 24 constitutes a compartment which can be partially or completely emptied as additional risers are added. Thus, in a first phase, the compartments are filled with an appropriate fluid such as water. Then, once the first riser has been fitted, part of a compartment is emptied and filled with gas, the amount emptied being a function of the weight of the riser fitted. This procedure then continues sequentially and in the same way for the other risers.

According to the described embodiment of the invention, the risers 11 are connected to the associated flexible pipes 10 by end fitting connections in a way known per se. These risers 11 are supported by the buoy by a connection and suspension receptacle device shown schematically in FIG. 1 and referenced 30, in which their terminal end fitting is housed. It can be noted that this device may comprise damping means intended to allow the risers a certain angular excursion with respect to the buoy at their connection. 

1-10. (canceled)
 11. A riser system for connecting a fixed subsea installation to a floating surface unit, comprising: a submerged buoy; at least one flexible pipe arranged in a first catenary and extending between the surface installation and the submerged buoy; at least one riser also arranged in a respective second catenary and extending between the buoy and the seabed; a tether device anchoring the buoy to the seabed, the tether device comprising at least two taut tethers between the buoy and the seabed; at least two moorings attached to the buoy and each in a third catenary; a respective return device on each third catenary positioned to exert a return force on the buoy that is dependent on the lateral movement of the buoy with respect to the seabed.
 12. A riser system according to claim 11, wherein each mooring has a lower portion toward the seabed, and the return device comprises at least one ballast weight arranged on the lower portion of each mooring.
 13. A riser system according to claim 12, wherein each mooring catenary has an end at the seabed and has a touchdown point spaced from the end at which the mooring touches down on the seabed when the buoy is not displaced from a first lateral position; and the ballast weight on the mooring is distributed on each side of the mooring from the touchdown point.
 14. A riser system according to claim 13, wherein a part of the ballast weight is situated above the touchdown point of the mooring and provides the return force on the buoy.
 15. A riser system according to claim 11, further comprising a bridle connecting each mooring to the buoy, the bridle being arranged under the buoy and the bridle is positioned for preventing the buoy from rotating.
 16. A riser system according to claim 11, wherein the taut tethers lie approximately in the mid-plane passing through a longitudinal axis of the buoy.
 17. A riser system according to claim 11, wherein the buoy is of variable buoyancy.
 18. A riser system according to claim 17, wherein the buoy is comprised of several parts each selectively operable to be buoyant, whereby the buoy is of variable buoyancy.
 19. A riser system according to claim 12, wherein the buoy has opposite lateral ends; and each of the moorings is connected to one lateral end of the buoy and the riser is connected to the other lateral end.
 20. A riser system according to claim 12, wherein each mooring is connected to the submerged buoy at a point lying approximately on the midplane of the buoy.
 21. A riser system for connecting a fixed subsea installation to a floating surface unit, comprising: a submerged buoy; at least one flexible pipe arranged in a first catenary and extending between the surface installation and the submerged buoy; at least one riser also arranged in a respective second catenary and extending between the buoy and the seabed; a tether device anchoring the buoy to the seabed; a mooring attached to the buoy and in a third catenary; a respective return device on the third catenary positioned to exert a return force on the buoy that is dependent on the lateral movement of the buoy with respect to the seabed.
 22. A riser system according to claim 21, wherein the mooring has a lower portion toward the seabed, and the return device comprises at least one ballast weight arranged on the lower portion of the mooring.
 23. A riser system according to claim 22, wherein the mooring catenary has an end at the seabed and has a touchdown point spaced from the end at which the mooring touches down on the seabed when the buoy is not displaced from a first lateral position; and the ballast weight on the mooring is distributed on each side of the mooring from the touchdown point.
 24. A riser system according to claim 23, wherein a part of the ballast weight is situated above the touchdown point of the mooring and provides the return force on the buoy.
 25. A riser system according to claim 21, wherein the buoy is of variable buoyancy.
 26. Method for producing a riser system between a subsea installation and a floating surface installation and for restoring a position of the floating surface installation, the method comprising: submerging a buoy, connecting the buoy to the surface installation by at least one flexible pipe and selecting the pipe length and the position of the buoy so that the pipe is arranged in a first catenary; anchoring the buoy to the seabed by a taut tether; arranging at least one riser in such a position and of such length as to define a second catenary between the buoy and the seabed, and connecting the riser to the at least one flexible pipe; exerting on the buoy a return force which depends upon the lateral movement of the buoy from a predetermined lateral position.
 27. The method of claim 21, further comprising attaching a mooring between the buoy and the seabed and forming the mooring of a length and attaching the mooring to the seabed such that a third catenary is defined; and wherein the return force is exerted by applying the return force on the mooring attached to the buoy, wherein the return force on the mooring moves the buoy laterally.
 28. The method of claim 22, wherein the mooring meets the seabed at a touchdown point, and the return force is applied increasingly as the mooring is lifted from the touchdown point on the seabed by lateral movement of the buoy. 